248 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



In summer the amount in the air is highest, in winter lowest. In 

 large coal-burning towns it is considerably more abundant than in the 

 country, and is deposited with carbonaceous, sulphurous, and organic 

 matter on exposed surfaces during the prevalence of fogs. Foggy air 

 in these towns contains an excess of sulphates and chlorides, but a 

 still greater excess of organic matter and ammonia salts, often double 

 the normal. The ammonia contained in the deposit on glass roofs in 

 Chelsea and Kew after fogs was respectively 1.4 and 1.1 per cent. The 

 processes of combustion, both in manufactories and in domestic fires, 

 of coal and of coal gas, give rise to ammonia. 



Only traces of ammonia are evolved from the lungs, and a little from 

 the skin and in perspiration. 



The smell of ammonia is distinguishable in most stables, but where 

 strong we may be sure that ventilation is deficient. Main streets, 

 especially where wooden pavements are used, often smell offensively of 

 ammonia; on still, dry days the ammoniacal dust is thick in the air, 

 and in windy weather is blown about in clouds. Analysis has shown 

 that 95 per cent of the dust from wooden pavements in main London 

 thoroughfares, consists of horse dung. This is breathed into the lungs 

 and often produces sore eyes and sore throat. Such pavements should 

 either be kept scrupulously cleau throughout the day or be properly 

 watered, in order to reduce harmful dust, and an occasional coating of 

 tar would not only prevent the emanation of noxious matter, but would 

 preserve the wood. 



Ammonia, being every where present in the air and extremely soluble 

 in water, may truly be said to be attached to all exposed surfaces where 

 moisture is also present; in the neighborhood of human habitations 

 and decaying animals or vegetable matter it has been found on all 

 objects; in a room, if a perfectly clean glass be suspended, traces of it 

 appear after an hour and a half. Evolved in small quantities from the 

 skin and lungs, it must be deposited with condensed vapors on the 

 walls, ceilings, and floors of dwelling houses. 



NITRIC ACID IN THE AIR. 



Mtric acid also pervades the air in minute quantity, and, with ammo- 

 nia, plays a great part in the development of plants. It results partly 

 from the combination of nitrogen and oxygen in the atmosphere caused 

 by thunder storms and partly by the oxidation in loamy soil of the 

 ammonia of decomposing organic matter. It seems probable that many 

 forms of bacteria or molds may be favored in their growth by the 

 presence, with moisture, of these two nitrogenous substances. Within 

 human habitations, cow sheds, etc., we must regard the walls, and all 

 surfaces as covered with a thin top-dressing of moist organic dust and 

 ammonia. Within the soil ammonia appears to be oxidized to nitrites 

 by one set of microorganisms, while another set oxidizes nitrites to 

 nitrates. To the latter the presence of ammonia is a hindrance. 



